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Abstract:

A developing device includes a developing sleeve for carrying a developer
containing a magnetic carrier and a non magnetic toner; a magnet,
provided in the sleeve and including a plurality of magnetic poles
disposed along a circumferential direction of the sleeve, for carrying
the developer on the sleeve; and a regulating member, provided opposed to
the sleeve with a predetermined spacing in a region in which the magnetic
poles different in polarity are adjacent to each other, for regulating an
amount of the developer carried on the sleeve. The magnetic poles are
disposed so that a circumferential direction component of a magnetic
force acting on the magnetic carrier contacting at least a part of an
upstream regulating surface of the regulating member with respect to the
circumferential direction of rotation of the sleeve is opposite from the
circumferential direction of the rotation.

Claims:

1. A developing device comprising: a developer carrying member for
carrying a developer containing a magnetic carrier and a non-magnetic
toner and for developing an electrostatic latent image formed on an image
bearing member; a magnet, provided in said developer carrying member and
including a plurality of magnetic poles disposed along a circumferential
direction of said developer carrying member, for carrying the developer
on said developer carrying member; and a regulating member, provided
opposed to said developer carrying member with a predetermined spacing in
a region in which the magnetic poles different in polarity are adjacent
to each other, for regulating an amount of the developer carried on said
developer carrying member, wherein the magnetic poles are disposed so
that a circumferential direction component of a magnetic force acting on
the magnetic carrier contacting at least a part of an upstream regulating
surface of said regulating member with respect to the circumferential
direction of rotation of said developer carrying member is opposite from
the circumferential direction of the rotation.

2. A developing device according to claim 1, wherein said regulating
member is disposed with a half-width of a peak position of the magnetic
force acting on the magnetic carrier with respect to the rotational
direction of said developer carrying member.

3. A developing device according to claim 1, wherein the magnetic poles
includes a first magnetic pole closest to said regulating member, a
second magnetic pole provided adjacent to and downstream of the first
magnetic pole with respect to the rotational direction of said developer
carrying member and a third magnetic pole adjacent to and upstream of the
first magnetic pole with respect to the rotational direction of said
developer carrying member, and wherein the second magnetic pole and the
third magnetic pole are different in polarity from the first magnetic
pole.

4. A developing device according to claim 3, wherein a maximum of a
component of the magnetic force, with respect to a radial direction of
said developer carrying member, acting on the magnetic carrier between a
peak position of a magnetic flux density of the first magnetic pole and a
peak position of a magnetic flux density of the third magnetic pole is
larger than a component of the magnetic force, with respect to the radial
direction of said developer carrying member, acting on the magnetic
carrier between the peak position of the magnetic flux density of the
first magnetic pole and a peak position of a magnetic flux density of the
second magnetic pole.

5. A developing device according to claim 3, wherein a magnetic flux
density gradient between the first magnetic pole and the third magnetic
pole is larger than a magnetic flux density gradient between the first
magnetic pole and the second magnetic pole.

6. A developing device according to claim 3, wherein on a surface of said
developer carrying member, an interval between a peak position of a
magnetic flux density of the first magnetic pole and a peak position of a
magnetic flux density of the third magnetic pole with respect to the
rotational direction of said developer carrying member is smaller than an
interval between the peak position of the magnetic flux density of the
first magnetic pole and a peak position of a magnetic flux density of the
second magnetic pole with respect to the rotational direction of said
developer carrying member.

7. A developing device according to claim 3, wherein a peak value of a
magnetic flux density of the third magnetic pole is larger than a peak
value of a magnetic flux density of the second magnetic pole.

8. A developing device according to claim 3. wherein a half-width of a
magnetic flux density of the third magnetic pole is larger than a
half-width of a magnetic flux density of the second magnetic pole.

9. A developing device according to claim 1, wherein the magnetic force
acting on the magnetic carrier contacting at least the part of the
upstream regulating surface of said regulating member with respect to the
rotational direction of said developer carrying member is larger than
1.0.times.10-0 (N).

10. A developing device according to claim 9, wherein the developer has
an angle of repose of 25 degrees to 50 degrees.

Description:

FIELD OF THE INVENTION AND RELATED ART

[0001] The present invention relates to a developing device for forming a
visible image by depositing a developer on an electrostatic latent image
formed an image bearing member.

[0002] The developing device is, e.g., used in an image forming apparatus,
of an electrophotographic type or an electrostatic recording type, such
as a copying machine, a laser beam printer, a facsimile machine or a
malfunction machine of these machines. In a conventional image forming
apparatus of the electrophotographic type, in general, a surface of a
drum-like photosensitive member which is the image bearing member is
electrically charged uniformly by a charger and then the charged
photosensitive member is exposed to light depending on image information
by an exposure device to form the electrostatic latent image on the
photosensitive member. The electrostatic latent image formed on the
photosensitive member is visualized as a toner image by a toner as the
developer by using the developing device. Then, the visualized image is
transferred onto a recording material by a transfer device. Thereafter,
the toner image transferred on the recording material is melt-fixed on
the recording material under heat and pressure by fixing device.

[0003] As such a developing device, there is a developing device using, as
the developer, a two-component developer including non-image toner
particles (toner) and magnetic carrier particles (carrier). Particularly,
in a color image forming apparatus, the toner may contain no magnetic
material and therefore the two-component developer has been widely used
for the reason such that color (tilt) is good or the like. In the
developing device using this two-component developer, in a developing
container, the toner and the carrier are fed while being stirred and then
the developer is carried on a developing sleeve as a developer carrying
member. The developer carried on the developing sleeve is regulated in
carrying amount by a regulating blade as a regulating member. Thereafter,
a developing bias is applied between the developing sleeve and the
photosensitive member and thereby only the toner is transferred onto the
electrostatic latent image formed on the photosensitive member surface,
so that the toner image corresponding to the electrostatic latent image
is formed on the photosensitive member surface.

[0004] The above-described developing device includes, as shown in FIG.
12, a magnet (multi-pole magnet) 45 as a magnetic field generating means
provided in a rotating developing sleeve 44. The magnet 45A has a
plurality of magnetic poles N1, N2, S1 and S2. Incidentally, a
rectilinear line in a radial direction represented by a lead line for
each of the magnetic poles shows a peak position of a magnetic flux
density of each of the positions.

[0005] The developer stirred and fed in a developing container 41 of the
developing device is scooped at an N2 pole (scooping pole) to be carried
on an outer peripheral surface of the developing sleeve 44. Then, by
rotation of the developing sleeve 44, as indicated by an arrow A, the
developer is fed to a developer stagnating portion 48 and an amount
thereof is regulated by a developer returning member 47. Then, in order
to constrain a stabilized developer, the developer is sufficiently
constrained at an S2 pole (cut pole) having a magnetic flux density not
less than a certain value and then is conveyed by a developing sleeve 44
while forming a magnetic chain.

[0006] Then, the developer carried on the developing sleeve 44 is
regulated in its amount by cutting the magnetic chain by a regulating
blade 46. The developer regulated in carrying amount is conveyed to an S1
pole (developing pole), which is an opposing portion to the
photosensitive member, via an N1 pole and then develops the electrostatic
latent image on the photosensitive member surface with the toner as
described above. The developer remaining on the surface of the developing
sleeve 44 after the development is separated from the developing sleeve
44 between an N3 pole and the N2 pole which are repelling poles and then
is collected in the developing container 41.

[0007] As described above, in the case of a structure in which the
developer carried on the developing sleeve 44 is regulated in amount by
the regulating blade, the developer which cannot pass through a spacing
(gap) between the developing sleeve 44 and the regulating blade 46 can
form an immobile layer. That is, in the case of the conventional
developing device, the magnet 45A was constituted so that a force in a
direction (arrow B direction) toward the regulating blade 46 acted on the
magnetic carrier upstream of the regulating blade 46 with respect to a
circumferential direction of rotation (rotational direction) of the
developing sleeve 44. For this reason, the developer which cannot pass
through the gap and stagnates upstream of the regulating blade 46 as
described above is pressed against an upstream surface of the regulating
blade 46, so that the immobile layer in which there is no motion of the
developer (or motion of the developer is less than that of another
portion) is liable to be formed.

[0008] Thus, when the immobile layer is formed on the upstream surface of
the regulating blade 46, the developer is rubbed at a boundary surface
between the immobile layer and a layer (flowable layer) of the developer
carried and conveyed by the developing sleeve 44. As a result, e.g., the
toner is liberated from the carrier by the rubbing and then the liberated
toner particles are liable to be adhered to each other by frictional heat
due to further rubbing, thus forming the immobile layer of the toner. The
thus formed immobile layer grows by the rotation of the developing sleeve
44, so that a gap between the immobile layer and the developing sleeve 44
becomes smaller than the gap between the regulating blade 46 and the
developing sleeve 44. Then, the carrying amount of the developer carried
and conveyed on the developing sleeve 44 is regulated by the gap between
the immobile layer and the developing sleeve 44, so that the carrying
amount of the developer becomes smaller than a set amount. As a result,
an amount of the developer conveyed to a developing region where the
developing sleeve 44 opposes the photosensitive member fluctuates, so
that a density of the image to be formed is lowered or density
non-uniformity occurs.

[0009] In order to prevent the formation of such an immobile layer, there
is a structure in which a cylindrical (columnar) toner feeding member
which rotates with a certain gap with a developing sleeve is provided
upstream of a regulating blade (Japanese Laid-Open Patent Application
(JP-A) Hei 5-35067). Further, there is also a structure in which a
regulating blade is provided between repelling poles in order to
uniformize a thickness of a layer of a developer carried on a developing
sleeve (JP-A Hei 5-6103).

[0010] In the case of the structure described in JP-A Hei 5-35067, a
bearing for supporting the toner feeding member and a driving means for
driving the toner feeding member are required, so that it is inevitable
that a constitution is complicated and a cost therefor is increased. In
addition, the toner feeding member is driven in an opposite direction at
a position where it opposes the developing sleeve and therefore strong
stress is imposed on the developer, so that there is a possibility that
the developer is deteriorated early. Further, in the case where the toner
feeding member is rotated at high speed, due to heat generation, there is
also a possibility that the developer is melted or adhered.

[0011] Further, in the case of the structure described in JP-A Hei 5-6103,
the regulating blade is provided between the repelling magnetic poles and
therefore an amount of the developed constrained in the neighborhood of
the regulating blade is small, so that there is a possibility that it
becomes difficult to stably supply the developer to the developing
sleeve.

SUMMARY OF THE INVENTION

[0012] A principal object of the present invention is to provide a
developing device capable of suppressing deterioration of a developer and
stably supplying the developer to a developer carrying member and also
capable of stably regulating an amount of the developer, carried on the
developer carrying member, by a regulating member.

[0013] According to an aspect of the present invention, there is provided
a developing device comprising: a developer carrying member for carrying
a developer containing a magnetic carrier and a non-magnetic toner and
for developing an electrostatic latent image formed on an image bearing
member; a magnet, provided in the developer carrying member and including
a plurality of magnetic poles disposed along a circumferential direction
of the developer carrying member, for carrying the developer on the
developer carrying member; and a regulating member, provided opposed to
the developer carrying member with a predetermined spacing in a region in
which the magnetic poles different in polarity are adjacent to each
other, for regulating an amount of the developer carried on the developer
carrying member, wherein the magnetic poles are disposed so that a
circumferential direction component of a magnetic force acting on the
magnetic carrier contacting at least a part of an upstream regulating
surface of the regulating member with respect to the circumferential
direction of rotation of the developer carrying member is opposite from
the circumferential direction of rotation.

[0014] These and other objects, features and advantages of the present
invention will become more apparent upon a consideration of the following
description of the preferred embodiments of the present invention taken
in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a schematic illustration of an image forming apparatus
according to First Embodiment of the present invention.

[0016] FIG. 2 is a schematic view for illustrating a measuring method of
an angle of response.

[0017]FIG. 3 is a schematic sectional view of a developing device in
First Embodiment.

[0018]FIG. 4 is a partly cut view of the developing device as seen from
above the developing device shown in FIG. 3.

[0019]FIG. 5 is a partly enlarged view, of a part of FIG. 3, showing a
two-dimensional distribution of fθ, with respect to a radial
direction, in the neighborhood of a regulating blade in First Embodiment.

[0020]FIG. 6 is a graph showing a relationship between a magnetic flux
density and a magnetic force in the neighborhood of the regulating blade
in First Embodiment.

[0021] FIG. 7 is a schematic view for illustrating definitions of Br,
Bθ, Fr and Fθ.

[0022]FIG. 8 is a schematic view showing a distribution of the magnetic
force and the magnetic flux density on a surface of a developing sleeve
in First Embodiment.

[0023]FIG. 9 is a partly enlarged view, of a part of FIG. 3, for
illustrating a flow of the developer upstream of the regulating blade.

[0024]FIG. 10 is a partly enlarged view, of a part of FIG. 3, showing a
developing device according to Second Embodiment of the present
invention.

[0025]FIG. 11 is a graph showing a relationship between a magnetic flux
density and a magnetic force in the neighborhood of a regulating blade in
Third Embodiment.

[0026]FIG. 12 is a schematic view for illustrating a conventional
developing device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiment 1

[0027] First, Embodiment of the present invention will be described with
reference to Figures to 9.

[Image Forming Apparatus]

[0028] First, a general structure and operation of an image forming
apparatus in this embodiment will be described with reference to FIG. 1.
An image forming apparatus 100 forms images in accordance with image
information from an original reading device connected to a main assembly
of the image forming apparatus 10 or from a host device, such as a
personal computer, which is communication connected to the main assembly.
In this embodiment, four color-based full-color image of yellow (Y),
magenta (M), cyan (C) and black (Bk) can be formed on a material (sheet
of recording paper, a sheet of plastic, piece of fabric, etc.) by using
an electrophotographic type.

[0029] Thus, the image forming apparatus 100 has a four-drum tandem type
constitution and includes, as a plurality of image forming means, first
to fourth image forming portions (image forming stations) PY, PM, PC and
PBk, which form yellow, magenta, cyan and black (monochromatic) images,
respectively. In a period in which an intermediary transfer belt 51
provided to a transferring device 5 as transferring means moves in an
indicated arrow direction and passes through the respective image forming
portions, the respective color toner images are superposed on the
intermediary transfer belt 51. Then, the multiple toner image superposed
on the intermediary transfer belt 51 are transferred onto the recording
material to obtain a recording image. In this embodiment, as the
developer, a two-component developer containing a non-magnetic toner and
a magnetic carrier is used.

[0030] Incidentally, the respective image forming stations have the
substantially same constitution except that they are different in
development color. Hereinafter, in the case where there is no need to
particularly differentiate the image forming station, suffixes Y, M, C
and Bk which indicate elements belonging to associated image forming
stations are omitted and will be collectively described.

[0031] The image forming station P includes a drum-like photosensitive
member 1 (photosensitive drum) as an image bearing member. Around the
photosensitive member 1, a charger 2 as a charging means, an exposure
device 3 as an exposure means (e.g., laser exposure optical system), a
developing device 4 as a developing means, the transferring device 5, a
cleaning device 7 as a cleaning means, and a charge removing device 8 as
a charge removing means are provided.

[0032] The transferring device 5 includes the intermediary transfer belt
51 as an intermediary transferring member. The intermediary transfer belt
51 is extended around a plurality of rollers, and is rotated (circularly
moved) in the direction indicated by the arrow in FIG. 1. Further, a
primary transferring member 52 is provided at a position where it opposes
an associated photosensitive member 1 via the intermediary transfer belt
51. Further, a secondary transferring member 52 is provided at a position
where it opposes one of the rollers around which the intermediary
transfer belt 51 is extended.

[0033] During image formation, first, the (peripheral) surface of the
rotating photosensitive member 1 is uniformly charged by the charger 2.
Next, the charged surface of the photosensitive member 1 is subjected to
scanning exposure by the exposure device 3 depending on an image
information signal, so that an electrostatic latent image is formed on
the photosensitive member 1 (image bearing member). The electrostatic
latent image formed on the photosensitive member 1 is visualized as a
toner image with the toner of the developer by the developing device 4.
At that time, depending on an amount of consumed toner, a supply
developer is supplied from a hopper 20 into the developing device 4
through an unshown supply path. The toner image formed on the
photosensitive member 1 is transferred (primary-transferred) onto the
intermediary transfer belt 51 by the action of a primary transfer bias
applied to the primary transferring member 52, at a primary transfer
portion (primary transfer nip), in which the intermediary transfer belt
51 and the photosensitive member 1 contact each other. For example,
during four color-based full-color image formation, the toner images are
sequentially transferred from the photosensitive members 1 of the for
image forming portions, starting from the first image forming portion RY
onto the intermediary transfer belt 51, so that a full (multi)-color
image consisting of the superposed four color toner image is formed on
the intermediary transfer belt 51.

[0034] Separately, the recording material accommodated in a cassette 9 is
conveyed by recording medium conveying members, such as a pickup roller,
conveyer rollers, registration rollers, and the like. This conveyance of
the recording material is effected in synchronism with the toner image on
the photosensitive member 1 at the second transfer portion (nip) where
the intermediary transfer belt 51 and a secondary transferring member 53
contact each other. Then, the multiple toner images on the intermediary
transfer belt 51 is transferred, at the secondary transfer portion, onto
the recording material, by the action of the secondary transfer bias
applied to the secondary transferring member 53.

[0035] Thereafter, the recording material is separated from the
intermediary transfer belt 51 is conveyed to a fixing device 6. The
counter images transferred on the recording material are subjected to the
heat and pressure applied thereto by the fixing device 6, thus being melt
and fixed on the recording material. Thereafter, the recording material
is discharged to the outside of the image forming apparatus 100.

[0036] After the primary transfer step, a deposited matter such as the
toner remaining on the photosensitive member 1 is collected by the
cleaning device 7. Further, the electrostatic latent image remaining on
the photosensitive member 1 is erased by the charge removing device 8. As
a result, the photosensitive member 1 is prepared for a subsequent image
forming step. Further, the deposited matter such as the toner remaining
on the intermediary transfer belt 51 after the secondary transfer step is
removed by an intermediary transfer belt cleaner 54.

[0037] Incidentally, the image forming apparatus 100 is also capable of
forming an image of a single color (e.g., black) or a multicolor image by
using the image forming portion for a desired single color or using two
or more of the four image forming stations for some colors.

[Two-Component Developer]

[0038] Next, the two-component developer used in this embodiment is
described. The toner contains colored particles made up of a binder
resin, a coloring agent, colored resin particles containing other
additives as desired, and external additives such as fine powder of
colloidal silica. Further, the toner is formed of a negatively chargeable
polyester resin material and is not less than 4.0 μm and not more than
1.0 μm in volume-average particle size d (4.0
μm-≦d≦10.0 μm), preferably be not less than 5.0 μm
and not more than 8.0 μm (5.0 μm≦d≦8.0 μm). In
this embodiment, d was 7.0 μm. In this embodiment, contains wax. The
toner contains the wax in an amount of 1-20 wt. %. For this reason, the
toner is obtained by kneading at least the binder resin, the coloring
agent and the wax, and then by pulverizing the kneaded product.

[0039] As the material for the carrier, surface-oxidized or non-oxidized
particles of a metallic substance, such as iron, nickel, cobalt,
manganese, chrome, rare-earth metal and their alloys, or oxidized
ferrite, and the like, can be suitably used. The method for manufacturing
these magnetic particles is not particularly limited. Further, the
carrier is 10.0 μm or more and 60.0μ or less, preferably be in a
range of 20.0-60.0μ, further preferably 30.0-50.0 μm, in
volume-average particle size D (10.0 μm≦D≦60.0 μm,
preferably 20.0μ≦D≦60.0 μm, further preferably, 30.0
μm≦D≦50.0 μm). Further, a volume resistivity is not
less than 107 Ω.cm, preferably not less than 108
Ω.cm and not more than 1014 Ω.cm. Further, an amount of
magnetization is 30 emu/cc (30×103 A/m) or more and 300 emu/cc
(300×103 A/m) or less. In this embodiment, the carrier which
was 40 μm in volume average particle size D, 5×108
Ω.cm in volume resistivity, and 260 μm/cc (260×103
A/m) in the amount of magnetization was used.

[0040] Incidentally, with respect to the toner, the volume-average
particle size was measured with the use of the following apparatus and
method. As the measuring apparatus, a Coulter Counter TA-AA (mfd. by
Beckman Coulter Inc.), an interface (mfd. by Nikkaki-Bios K.K.) for
outputting the number and volume average distributions of the developer,
and a personal computer ("CX-1", mfd. by Canon K.K.) were used. As an
electrolytic aqueous solution, 1% NaCl aqueous solution prepared by using
a first class grade sodium chloride was used.

[0041] The measuring method is as follows. That is, 0.1 ml of a
surfactant, preferably alkyl-benzene sulfonate, was added, as dispersant,
into 10-150 ml of above-mentioned electrolytic aqueous solution. Then,
0.5-50 mg of a measurement sample was added to the above mixture. Then,
the electrolytic aqueous solution in which the sample was suspended was
subjected to dispersion by an ultrasonic dispersing device for about 1-3
minutes. Then, the distribution of the particles which were in a range of
2-40 μm in diameter was obtained with the use of the Coulter Counter
TA-II fitted with a 100 μm aperture as an aperture. The volume-average
particle size was obtained from the thus obtained volume-average
distribution.

[0042] The resistivity of the magnetic carrier was measured in the
following manner. That is, a cell of the sandwich type, which was 4 cm in
the area (size) of each of its measurement electrodes, and was 0.4 cm in
the gap between the electrodes, was used. Then, the resistivity was
measured by a method in which the carrier resistivity was obtained from
electric current which flowed through a circuit while 1 kg of weight was
applied to one of the electrodes and a voltage E (V/cm) was applied
between the two electrodes. Further, the volume-average particle size of
the magnetic carrier was measured with the use of a particle size
distribution measuring device ("HERO", mfd. by JEOL Ltd.) of the laser
diffraction type (NEC Corp.), and the particle size range of 0.5-350
μm was, based on volume basis, logarithmically divided into 32
decades, and the number of particles in each decade was measured. Then,
from the results of the measurement, the median diameter of 50% in volume
was used as the volume-average particle size.

[0043] Further, the magnetic properties of the magnetic carrier were
measured with the use of an automatic magnetic property recorder of
vibratory (BHV-30 mfd. by Riken Denshi Co., Ltd.). As a magnetic
characteristic value, the magnetic (magnetization) strength of the
magnetic carrier was obtained by forming external magnetic fields, which
were 795.7 kA/m and 79.58 kA/m, respectively. A sample of the magnetic
carrier for measurement was prepared by packing the magnetic carrier in a
cylindrical plastic container so as to be sufficiently dense. In this
state, the magnetizing moment was measured and further, an actual weight
of the sample was weighed to obtain the strength of magnetization
(emu/g). Further, the true specific gravity of the magnetic carrier
particles was obtained with the use of, e.g., a micromeritics gas
pycnometer ("AccuPyc 1330, mfd. by Shimazu Corp.) (which is an automatic
densitometer of the dry type), or the like. The strength of magnetization
per unit volume was obtained by multiplying the obtained strength of
magnetization (per unit volume) by the true specific gravity.

[0044] Next, a degree of agglomeration (agglomerativeness) of the
developer will be described. Here, the degree of agglomeration of the
developer can be measured in terms of an angle of repose. A proper range
of the angle of repose of the developer in this embodiment is 25-50
degrees, preferably 30-45 degrees. When the angle of repose of the
two-component developer is smaller than 25 degrees, due to high
flowability, problems of scattering and white dropout during transfer on
the plurality of sheets of the recording material occur and a transfer
property during a durability test (when printing on a large number of
sheets is effected) cannot be sufficiently retained satisfactorily.
Further, when the angle of repose is larger than 50 degrees, the levels
of the scattering and white dropout at an initial printing state are good
but during the durability test at high speed, the developing property is
lowered and a load of the feeding screw is increased, thus leading to
screw locking. Therefore, in this embodiment, the two-component developer
with the angle of repose of 40 degrees is used.

[0045] FIG. 2 is a schematic view for illustrating an example of a method
of measuring the angle of repose. In this embodiment, an angle of repose
φ of the toner was measured by using the following method. First, a
measuring apparatus is a powder tester ("PT-N", mfd. by Hosokawa Micron
Corp.). Further, the measuring method is in accordance with measurement
of the angle of repose in an operation manual attached to the powder
tester (PT-N) (aperture of sieve 301: 710 μm, vibration time: 180 s,
amplitude: 2 mm or less). The developer is dropped from a funnel 303 onto
a disk 302, and an angle formed between a generating line of a developer
500 deposited in a conical shape on the disk 302 and the surface of the
disk 302.

[0046] However, the sample is left standing overnight in an environment of
23° C. and a relative humidity of 60% (i.e., 60% RH) and then the
angle of repose is measured and repeated five times in the measuring
apparatus in the environment of 23° C. and 60% RH. An arithmetic
average of the five measured values is used as φ.

[Developing Device]

[0047] Next, referring to FIGS. 3 and 4, the developing device 4 is
described. The developing device 4 includes a developing container 41, in
which the two-component developer containing the toner and the carrier is
accommodated. The developing device 4 also includes, at a position where
the developing container 41 opposes the photosensitive member 1, a
developing sleeve 44 as a developer carrying means, and a regulating
blade 46 as a regulating member for regulating a thickness of the chain
of the developer carried on the developing sleeve 44.

[0048] Further, the inside of the developing container 41 is partitioned
into a developing chamber 41a and a stirring chamber 41b by a partition
wall 41c which extends in the direction perpendicular to the surfaces of
the drawing sheets of FIGS. 2 and 3.

[0049] In the developing chamber 41a and stirring chamber 41b, first and
second feeding screws 42 and 43 are provided, respectively, as a
developer feeding member. The first feeding screw 42 is provided at the
bottom of the developing chamber 41a and is substantially parallel to the
axial direction of the developing sleeve 44, and conveys, while stirring,
the developer in the development chamber 41a in one direction along the
axial direction of the developing sleeve 44, by being rotated in the
direction (clockwise direction) indicated by an arrow in FIG. 3. The
reason why the first feeding screw 42 is rotated in the clockwise
direction is that the clockwise direction is advantageous from the
standpoint of supplying the developing sleeve 44 with the developer.
Further, the second feeding screw 43 is provided at the bottom of the
stirring chamber 41b and is substantially parallel to the first feeding
screw 42, and conveys, while stirring, the developer in the stirring
chamber 41b in a direction opposite from that by the first feeding screw
42 by being rotated in the opposite direction (counterclockwise
direction) from the rotational direction of the first feeding screw 42.

[0050] Thus, by rotation for stirring of the first and second feeding
screws 42 and 43, the developer is circulated between the developing
chamber 41a and the stirring chamber 41b through openings (communicating
portions) 41d and 41e passages at longitudinal ends of the partition wall
41c.

[0051] The developing container 41 is provided with an opening at a
position corresponding to the developing region α where the
positioning container 41 opposes the photosensitive member 1. At this
opening, the developing sleeve 44 is rotatably provided so as to be
partly exposed toward the photosensitive member 1. Further, the
developing sleeve 44 and the photosensitive member 1 are brought near to
and opposed to each other. For example, it is assumed that the developing
sleeve 44 and the photosensitive member 1 are 20 mm and 80 mm,
respectively, in diameter, and the closest distance therebetween is about
300 μm. As a result, setting is made so that the development can be
effected in a state in which the developer fed by developing sleeve 44 to
the developing region α is brought into contact with the
photosensitive member 1.

[0052] Such a developing sleeve 44 is constituted in a cylindrical
(columnar) shape by a nonmagnetic material such as aluminum or stainless
steel. Inside the developing sleeve 44 (developer carrying member), a
cylindrical magnet 45 which is a multi-pole magnet is provided in a
stationary (non-rotational state). This magnet 45 has a plurality of
magnetic poles disposed in its circumferential direction. Specifically,
with respect to the rotational direction (arrow direction or clockwise
direction) of the developing sleeve 44, the magnetic poles are arranged
in the order of S1 pole as a developing disposed opposed to the magnet 1
in the developing region α, an N3 pole, an N2 pole, an S2 pole and
an N1 pole. Incidentally, in FIG. 3 and in FIGS. 5, 9 and 11 described
later, a rectilinear line extending in the radial direction indicated by
a lead line for each magnetic pole represents a peak position of a
magnetic flux density of each magnetic pole.

[0053] During the development, the developing sleeve 44 is rotated (i.e.,
the developer is carried and conveyed) in a state in which the developer
is carried on the developing sleeve 44 by a magnetic attraction force.
The developing sleeve 44 carries the two-component developer regulated in
layer thickness by cutting of the chain of the magnetic brush with the
regulating blade 46 and conveys the developer to the developing region a
in which the developing sleeve 44 opposes the photosensitive member 1.
Then, the developing sleeve 44 supplies the developer to the
electrostatic latent image formed on the photosensitive member 1 to
develop the electrostatic latent image.

[0054] At this time, in order to improve a developing efficiency, i.e., a
degree of impartment of the toner to the electrostatic latent image, a
developing bias voltage in the form of a DC voltage biased (superposed)
with an AC voltage is applied from a power source to the developing
sleeve 44. In this embodiment, the DC voltage of -500 V and the AC
voltage of 800 V in peak-to-peak voltage (Vpp) and 12 kHz in frequency
(f) were used. However, the DC voltage value and the AC voltage waveform
are not limited thereto. Further, in general, in a two-component magnetic
brush developing method, when the AC voltage is applied, the developing
efficiency is increased and thus the image is high in quality but is
rather liable to cause fog. For this reason, the fog is prevented by
providing a potential difference between the DC voltage applied to the
developing sleeve 44 and a charge potential of the photosensitive member
1 (i.e., a white background portion potential).

[0055] In the developing region α, the developing sleeve 44 of the
developing device 4 is rotated together with the photosensitive member 1
in the same direction as that of the photosensitive member 1, and a
peripheral speed ratio of the developing sleeve 44 to the photosensitive
member 1 is 1.75. The peripheral speed ratio may be set in a range of
0.5-2.5, preferably 1.0-2.0. When the movement (peripheral) speed ratio
is larger, the developing efficiency is correspondingly increased.
However, when the ratio is excessively large, problems of toner
scattering developer deterioration and the like occur and therefore the
peripheral speed ratio may preferably be set in the above-described
ranges.

[0056] Further, the regulating blade 46 which is the regulating member
(chain cutting member) is constituted by a non-magnetic member formed of
aluminum or the like in a plate shape extending along a longitudinal
axial line direction of the developing sleeve 44, and is provided
upstream of the photosensitive member 1 with respect to the developing
sleeve rotational direction. Further, the regulating member 46 is
disposed opposed to the developing sleeve 44, thus regulating the amount
of the developer carried on the developing sleeve 44. Then, both of the
toner and the carrier which constitute the developer pass through the gap
between an end of the regulating blade 46 and the developing sleeve 44 to
be sent to the developing region α.

[0057] Incidentally, by adjusting the spacing (gap) between the end of the
regulating blade 46 and the surface of the developing sleeve 44, a
cutting amount of the chain of the magnetic brush of the developer
carried on the developing sleeve 44 is regulated, so that the amount of
the developer conveyed to the developing region α is adjusted. For
example, a coating amount per unit area of the developer on the
developing sleeve 44 is regulated at 30 mg/cm2 by the regulating
blade 46. Incidentally, the gap between the regulating blade 46 and the
developing sleeve 44 is set at 200-1000 μm, preferably 300-700 μm.
In this embodiment, the gap was set at 500 μm.

[Magnet (Multi-Pole Magnet)]

[0058] Next, the magnet 45 in this embodiment will be described with
reference to FIGS. 5 to 9. A magnetic flux density relationship among the
plurality of magnetic poles of the magnet 45 is set so that a
circumferential (rotational) direction component (fθ) of the
magnetic force acts, in the direction opposite from the rotational
direction of the developing sleeve 44, on the magnetic carrier contacting
the regulating blade 46 at the upstream side of the regulating blade 46
with respect to the rotational direction of the developing sleeve 44.
That is, the magnetic flux density relationship among the plurality of
magnetic poles is set so that the direction of the developing sleeve 44
rotational direction component of the magnetic force acting on the
magnetic carrier contacting the upstream side of the regulating blade 46
with respect to the rotational direction of the developing sleeve 44 is
opposite from the rotational direction of the developing sleeve 44.
Incidentally, the rotational direction of the developing sleeve 44
indicated by an arrow C in each of FIGS. 3, 5, 6, 7, 9, 10 and 11 is
hereinafter referred to as a sleeve rotational direction.

[0059] Specifically, as shown in FIG. 5, the magnet 45 has the S2 pole and
the N1 pole which are mutually different in polarity and are disposed in
the neighborhood of the regulating blade 46. Of these poles, the S2 pole
(cut pole) as a first magnetic pole has a magnetic flux density peak
value at a position which is upstream of the regulating blade 46 with
respect to the sleeve rotational direction and is closest to the
regulating blade 46. Further, the N1 pole as a second magnetic pole has a
magnetic flux density peak value at a position which is downstream of the
regulating blade 46 with respect to the sleeve rotational direction and
is closest to the regulating blade 46. Further, the magnet 45 has the N2
pole, as a third magnetic pole, which is disposed adjacent to and
upstream of the S2 pole and which is different in polarity from the S2
pole.

[0060] In this embodiment, in order to direct the rotational direction
component (Fθ) of the magnetic force in the direction opposite from
the rotational direction of the developing sleeve 44, as a basic way of
thinking, the magnetic flux density relationship may be designed in the
following manner. That is, with respect to the magnetic pole closest to
the regulating blade 46 (i.e., the cut pole (S2 pole in FIG. 5)), the
intensity of the magnetic flux density of each of the magnetic poles N2
and N1 upstream and downstream of the cut pole with respect to the sleeve
rotational direction and an interval between the cut pole and each of the
magnetic poles N2 and N1 may be adjusted. Specifically, the magnetic
force of the upstream magnetic pole N2 acting on the cut pole S2 may be
made intenser than the magnetic force of the downstream magnetic pole N1
acting on the cut pole S2. As a method therefor, when the magnetic flux
density of the magnetic pole immediately upstream of the cut pole is made
larger than the magnetic flux density of the magnetic pole immediately
downstream of the cut pole, the direction of the rotational direction
component (Fθ) of the magnetic force approaches the direction
opposite from the rotational direction of the developing sleeve 44.

[0061] Further, even in the case where the magnetic flux densities of the
magnetic poles upstream and downstream of the cut pole are the same, when
the magnetic pole upstream of the cut pole is brought near to the cut
pole, the direction of the rotational direction component (Fθ) of
the magnetic force can approach the direction opposite from the
rotational direction of the developing sleeve 44. Further, also when a
magnetic flux density half-width of the magnetic pole immediately
upstream of the cut pole is made narrower than a magnetic flux density
half-width of the magnetic pole immediately downstream of the cut pole,
the magnetic force can be increased. Further, also in this case, the
direction of the rotational direction component (Fθ) of the
magnetic force can be brought near to the direction opposite from the
rotational direction of the developing sleeve 44.

[0062] In this embodiment, as shown in FIG. 5, an sleeve rotational
direction interval between the magnetic flux density peak value of the S2
pole and the magnetic flux density peak value of the N2 pole is made
smaller than an sleeve rotational direction interval between the magnetic
flux density peak value of the S2 pole and the magnetic flux density peak
value of the N1 pole. Here, when the peak values and half-widths of the
magnetic flux densities of the N1 pole and the N2 poles between which the
S2 pole is interposed are the same, as described above, the interval
between the magnetic flux density peak values of the S2 pole and the N2
pole may be made smaller than the interval between the magnetic flux
density peak values of the S2 pole and the N1 pole. That is, when the
peak value intervals are regulated as described above, with respect to
the magnetic carrier contacting the upstream side of the regulating blade
46 with respect to the sleeve rotational direction, as indicated by
arrows D in FIG. 5, the rotational direction component of the magnetic
force acts in the direction opposite from the sleeve rotational
direction. Incidentally, a similar effect is obtained also when the peak
value or half-width of the magnetic flux density of the N2 pole is made
larger than the peak value or half-width of the magnetic flux density of
the N1 pole.

[0063] However, as shown in FIG. 6, even when the peak value or half-width
of the magnetic flux density of the N2 pole is smaller than the peak
value or half-width of the magnetic flux density of the N1 pole, the
interval between the magnetic flux density peak values of the S2 pole and
the N2 pole may be made smaller than the interval between the magnetic
flux density peak values of the S2 pole and the N1 pole so as to cause
the above-described magnetic force to act. That is, with respect to the
peak values and half-widths of the magnetic flux densities of the
magnetic poles N2, S2 and N1 in the neighborhood of the regulating blade
46, the intervals of these peak values are appropriately regulated. Then,
the rotational direction component of the magnetic force is caused to
act, in the direction opposite from the sleeve rotational direction, on
the magnetic carrier contacting the upstream side of the regulating blade
46 with respect to the sleeve rotational direction.

[0064] This point will be described with reference to FIG. 6 to FIG. 8.
FIG. 6 shows a relationship between the magnetic flux density and the
magnetic force with respect to the rotational direction (angle) of the
developing sleeve 44. Here, "Br" indicated by a (black) square mark is a
radial direction component of the magnetic flux density, "Fr" indicated
by a (black) circular mark is a radial direction component of the
magnetic force acting on the magnetic carrier, and "Fθ" indicated
by a line is a sleeve rotational direction component of the magnetic
force acting on the magnetic carrier. Further, in the case where Fr is
positive, the magnetic force acts in a divergent direction, i.e., a
direction in which the magnetic force is moved away from the sleeve. In
the case where Fθ is positive, the magnetic force acts in the
sleeve rotational direction. Incidentally, "Bθ" is a sleeve
rotational direction component of the magnetic force acting on the
magnetic carrier.

[0065] These components Br, Bθ, Fr and Fθ are defined as shown
in FIG. 7. That is, in the case where the radius of the developing sleeve
44 is R and an angle at an arbitrary point on the outer peripheral
surface of the developing sleeve 44 is Θ, at this arbitrary point,
the respective components Br, Bθ, Fr and Fθ act as indicated
as associated arrows. Each of the directions of the arrows represents a
positive direction. Further, the peak value and half-width of the
magnetic flux density in this embodiment represent those of the radial
direction component Br of the magnetic flux density.

[0066] In FIG. 6, the position of the regulating blade 46 is indicated by
a broken line, i.e., in the neighborhood of the angle of 80 degrees.
Further, at the left side of this broken line, i.e., at the upstream side
of the regulating blade 46 with respect to the sleeve rotational
direction, Fθ is negative, so that the magnetic force acting on the
magnetic carrier acts in the opposite direction to the sleeve rotational
direction. In this embodiment, in this way, the magnetic flux density
relationship among the respective magnetic poles is regulated as
described above so that Fθ is negative at the upstream side of the
regulating blade 46 with respect to the sleeve rotational direction.
Incidentally, Fθ becomes a positive value in the neighborhood of
the magnetic flux density peak value of the N2 pole and Fθ at this
position is indicated by arrows E.

[0067] Further, as shown in FIG. 8, in the case where the position of the
developing sleeve 44 is the position of 0 (zero) on the ordinate, it is
preferable that Fθ is negative at any position with respect to the
radial direction of the developing sleeve 44 upstream of the regulating
blade 46. Here, the ordinate in FIG. 8 represents the rotational
direction of the developing sleeve 44 and the abscissa represents the
radial direction of the developing sleeve 44. The position of 0 on the
abscissa is the surface of the developing sleeve 44. The magnetic flux
density is large at the surface of the developing sleeve 44 and is
smaller with a distance from the sleeve surface. For this reason,
depending on a magnetic flux density relationship between adjacent
magnetic poles, it would be considered that Fθ is positive at a
position distant from the sleeve surface. When Fθ is positive, as
described later, the developer is pressed against the regulating blade
46, so that the immobile layer is liable to be generated. Therefore, as
shown in FIG. 8, irrespective of the radial direction position of the
regulating blade 46, it is preferable that the magnetic flux density
relationship between the adjacent magnetic poles is regulated so that
Fθ is negative.

[0068] Here, a calculation method of the magnetic force represented by the
following formula will be described.

{right arrow over (F)}=(Fr,Fθ)

[0069] The magnetic force acting on the magnetic carrier is represented by
the following formula:

[0071] Therefore, when Br and Be are known, Fr and Fθ can be
obtained. Here, the magnetic flux density Br can be measured by using, as
a measuring device, a magnetic field measuring device ("MS-9902" (trade
name), mfd. by F.W. BELL, Inc.). For example, the magnetic flux density
Br is measured by setting a distance between a probe, which is a member
of the measuring device, and the surface of the developing sleeve 44 at
about 100 μm.

[0072] Further, Bθ can be obtained in the following manner. Vector
potential AZ (R, θ) at a measuring position of the magnetic
flux density Br is obtained by using the measured magnetic flux density
Br according to the following formula.

A z ( R , θ ) = ∫ 0 θ RBr
θ ##EQU00003##

[0073] Under a boundary condition of AZ (R, θ), AZ (r,
θ) is obtained by solving the following equation.

∇2A2(r,θ)=0

[0074] Then, Bθ can be obtained from the following equation.

B θ = - ∂ A z ( r , θ )
∂ r ##EQU00004##

[0075] Br and Bθ measured and calculated in the above-described
manner are applied to the above formulas, so that Fr and Fθ can be
derived.

[0076] As described above, in order to make the direction of Fθ
opposite from the sleeve rotational direction at the upstream position of
the regulating blade 46, a maximum of Fr (absolute value) between the S2
pole and the N2 pole is made larger than a maximum of Fr (absolute value)
between the S2 pole and the N1 pole. Here, Fr between the S2 pole and the
N2 pole is the radial direction component (Fr) of the magnetic force
acting on the magnetic carrier between the peak position of the magnetic
flux density (Br) of the S2 pole and the peak position of the magnetic
flux density (Br) of the N2 pole. Further, Fr between the S2 pole and the
N1 pole is the radial direction component (Fr) of the magnetic force
acting on the magnetic carrier between the peak position of the magnetic
flux density (Br) of the S2 pole and the peak position of the magnetic
flux density (Br) of the N1 pole.

[0077] Further, in order to regulate the values of Fr between the
respective magnetic poles, a gradient of the magnetic flux density
between the S2 pole and the N2 pole is made larger than a gradient of the
magnetic flux density between the S2 pole and the N1 pole. That is, the
gradient of Br between the adjacent two poles is increased, so that a
maximum of B2 is generated between the adjacent two poles and thus the
magnetic carrier is attracted to the direction in which B2 is large.
Therefore, by making the gradient of the change in Br between the S2 pole
and the N2 pole larger than the gradient of the change in Br between the
S2 pole and the N1 pole, the direction of Fθ can be made opposite
from the sleeve rotational direction at the position of the regulating
blade 46.

[0078] According to this embodiment, the magnetic force component
(Fθ) directed in the opposite direction to the sleeve rotational
direction acts on the magnetic carrier contacting the upstream side of
the regulating blade 46 and therefore the developer is not readily
pressed against the upstream surface of the regulating blade 46. As a
result, it is possible to suppress the generation of the immobile layer
at the surface without exerting strong stress on the developer, so that
the carrying amount of the developer carried on the developing sleeve 44
can be stably regulated by the regulating blade 46.

[0079] This point will be described with reference to FIG. 9. The
developer carried and conveyed by the developing sleeve 44 is supplied to
the developer stagnating portion 48. Then, the developer is, after
striking against the regulating blade 46, divided into the developer
which passes through the gap between the regulating blade 46 and the
developing sleeve 44 and the developer which cannot pass through the gap
between the regulating blade 46 and the developing sleeve 44 and remains
at the developer stagnating portion 48. The stagnated developer has
nowhere to go and then moves, in the neighborhood of the regulating blade
46, in a direction in which the developer is spaced from the developing
sleeve 44 (in the upward direction in FIG. 9). Thereafter, motion of the
developer at the developer stagnating portion 48 is determined by the
rotational direction component Fθ of the magnetic force acting from
the magnet 45.

[0080] In this case, when the direction of the rotational direction
component Fθ of the magnetic force acting from the magnet 45 at the
surface at the developer stagnating portion 48 side of the regulating
blade 46 is the same as the sleeve rotational direction is pressed
against the regulating blade 46.

[0081] Then, the developer forms the immobile layer to liable to result in
the toner layer. On the other hand, the direction of the rotational
direction component Fθ of the magnetic force acting from the magnet
45 at the developer stagnating portion 48 side of the regulating blade 46
is opposite from the sleeve rotational direction, the developer, in the
neighborhood of the regulating blade 46, at the developer stagnating
portion 48 mores in the develop to the sleeve rotational direction. As a
result, it is possible to suppress the generation of the immobile layer
at the developer stagnating portion, so that the generation of the toner
immobile layer can be suppressed.

[0082] Incidentally, in this case, the motion of the developer at the
developer stagnating portion 48 can be predicted so that the developer
moves along an arrow F direction shown in FIG. 9. This also coincided
with an observation result in an experiment conducted by the present
inventors. Thus, by the movement of the developer at the upstream
position of the regulating blade 46, the generation of the immobile layer
is readily suppressed. Particularly, when Fθ directed in the
opposite direction to the sleeve rotational direction is large at the
position of the regulating blade 46, the motion of the developer at the
developer stagnating portion 48 becomes active, thus being suitable in
terms of the suppression of the generation of the immobile layer.

[0083] Next, a preferred range of Fθ in order to suppress the
developer immobile layer was obtained in the following manner. With
respect to the S2 pole closest to the regulating blade 46 when the amount
of magnetization of the carrier is minimum (30 emu/cc), the intensity,
half-width and interval from the cut pole of each of the magnetic poles
N2 and N1 located upstream and downstream of the S2 pole (cut pole) with
respect to the developing sleeve rotational direction are changed, so
that Fθ changes its amplitude while becoming negative and then a
state of the generation of the immobile layer was checked. The result is
shown in Table 1. Further, in this case, the developer with the angle of
repose of 50 degrees at which the immobile layer was most generated was
used.

[0084] From Table 1, it was possible to confirm that there is no immobile
layer generation when Fθ is larger than 1.0×10-9 (N).

[0085] When the immobile layer generation can be suppressed, the amount of
the developer carried on the developing sleeve 44 can be properly
suppressed by the regulating blade 46, so that it is possible to
stabilize the carrying amount for a long term. As a result, it is
possible to suppress a fluctuation of the amount of the developer
conveyed to the developing region where the developing sleeve 44 opposes
the photosensitive member, so that a lowering in density of an image to
be formed and an occurrence of density non-uniformity can be reduced.

[0086] Further, in this embodiment, the S2 pole as the first magnetic pole
in the neighborhood of the regulating blade 46 and the N1 pole as the
second magnetic pole are different in polarity and therefore different
from the structure described in JP-A Hei 5-6103, the developer can be
stably supplied to the developing sleeve. Further, the magnetic flux
density relationship between the respective magnetic poles of the magnet
45 is only regulated and thus there is no need to provide another
particular member, so that it is possible to achieve the above effect at
a low cost.

[0087] Further, in this embodiment, the toner containing a wax is used.
With respect to this wax-containing toner, by the friction at a boundary
surface the immobile layer and the flowable layer, the wax having
viscosity is present at the toner surface. As a result, the toner
particles are liable to adhere to each other, so that there is a
possibility that a toner agglomerate is generated to fluctuate the
carrying amount of the developer on the developing sleeve 44. On the
other hand, in this embodiment, as described above, the immobile layer is
not readily generated and therefore it is possible to less generate the
toner agglomerate even when the toner contains the wax.

Second Embodiment

[0088] Second Embodiment of the present invention will be described with
reference to FIG. 10. In this embodiment, the peak values of the magnetic
flux densities (Br) of the N2 pole as the third magnetic pole and the N1
pole as the second magnetic pole which constitute the magnet 45 are set
so that the former peak value is larger than the latter peak value. Thus,
the rotational direction component (Fθ) of the magnetic force is
caused to act, in the opposite direction to the sleeve rotational
direction, on the magnetic carrier contacting the upstream side of the
regulating blade 46 with respect to the sleeve rotational direction.

[0089] Incidentally, the half-width of the magnetic flux density of the N2
pole may preferably be not less than the half-width of the magnetic flux
density of the N1 pole. Further, the sleeve rotational direction interval
between the peak values of the magnetic flux densities of the S2 pole and
the N2 pole may preferably be not more than the sleeve rotational
direction interval between the peak values of the magnetic flux densities
of the S2 pole and the 1 pole.

[0090] However, when the rotational direction component (Fθ) of the
magnetic force acts, in the develop to the sleeve rotational direction,
on the magnetic carrier contacting the upstream side of the regulating
blade 46 with respect to the sleeve rotational direction, it is possible
to appropriately set the above-described half-widths and peak value
intervals. That is, in this embodiment, with respect to these half-widths
and peak value intervals, by making the magnetic flux density peak value
of the N2 pole larger than the magnetic flux density peak value of the N1
pole, Fθ may only be required to be directed in the develop to the
sleeve rotational direction at the upstream position of the regulating
blade 46.

[0091] A specific setting example in this embodiment is shown in Table 2.

[0092] In the constitution of Table 2, the magnetic flux density peak
value of the N2 pole is two times the magnetic flux density peak value of
the N1 pole. On the other hand, the magnetic flux density half-widths of
the N2 pole and the N1 pole are the same, and the peak value interval
between the S2 pole and the N2 pole and the peak value interval between
the S2 pole and the N1 pole are also the same. Also in this embodiment,
the generation of the immobile layer can be suppressed. Other structures
and functions are the same as those in First Embodiment described above.

Third Embodiment

[0093] Third Embodiment of the present invention will be described based
on FIG. 11 while making reference to FIG. 3 and the like. In this
embodiment, the regulating blade 46 is disposed within a region, of the
region in which the rotational direction component of the magnetic force
acts in the opposite direction to the sleeve rotational direction, in
which the rotational direction component is larger than 1/2 of the
maximum. That is, the position of the regulating blade 46 indicated by
the broken line is located within a region β, of a region α in
which Fθ is negative in FIG. 11, in which the rotational direction
component is larger than 1/2 of the maximum of Fθ in term of the
absolute value. As a result, the rotational direction component (F
acting, in the opposite direction to the sleeve rotational direction, on
the magnetic carrier generating the upstream side of the regulating blade
46 with respect to the sleeve rotational direction can be made large, so
that the generation of the immobile layer can be more suppressed.

[0094] This embodiment is preferably applicable to a system in which a
carrier with a small amount of magnetization is used. First, the carrier
used in this embodiment will be described. In this embodiment, the
carrier of 40 μm in volume-average particle size, 5×108
Ω.cm in volume resistivity and 180 emu/cc in amount of
magnetization is used. A proper range of the amount of magnetization of
the magnetic carrier is 30-300 emu/cm3, preferably 100-280
emu/cm3. When the amount of magnetization is less than 30
emu/cm3, the amount of the carrier deposition on the photosensitive
member 1 is increased and in addition, magnetic application and
conveyance of the developer on the developing sleeve 44 cannot be
effected. When the amount of the magnetization is more than 300
emu/cm3, image non-uniformity due to the magnetic brush chain is
liable to occur.

[0095] Further, by optimizing the amount of magnetization of the magnetic
carrier and at the same time by optimizing the ranges of the particle
size and resistivity (specific resistance) of the magnetic carrier, it is
possible to prevent further reliably the carrier deposition and the image
deterioration. That is, when the number-average particle size of the
magnetic carrier falls within the range of 10-60 μm, the deposition of
the carrier with a small particle size on the photosensitive drum can be
prevented and sweep non-uniformity of the image by the carrier with a
large particle size can be made less visible. By setting the resistivity
of the carrier within the range of 107-1014 Ω.cm, even
with respect to the carrier with a low amount of magnetization, the
carrier deposition due to the electric charge injection can be prevented
and the image deterioration due to charge-up of the carrier can be
prevented.

[0096] Generally, when the carrier with the small amount of magnetization
is used, stress on the developer in the developing container 41 is
decreased, so that the lifetime extension can be achieved. Further, the
magnetic brush is soft and therefore the frictional force against the
photosensitive member 1 is decreased. For this reason, there is the
advantage that the toner subjected to the development is not disturbed
and thus high quality can be achieved. On the other hand, the magnetic
force acting from the magnet 45 on the magnetic carrier contacting the
upstream side of the regulating blade 46 with respect to the developing
sleeve rotational direction is decreased relative to that in the case of
using the carrier with a large amount of magnetization, so that the
motion of the developer tends to become slow. As a result, even in the
case where the rotational direction component Fθ of the magnetic
force acting from the magnet 45 on the magnetic carrier contacting the
upstream side of the regulating blade 46 with respect to the developing
sleeve rotational direction is directed in the opposite direction to the
sleeve rotational direction, when the magnitude of the rotational
direction component Fθ is excessively small, the immobile layer is
liable to generate.

[0097] Therefore, the direction the rotational direction component
Fθ acting from the magnet 45 on the magnetic carrier contacting the
upstream side of the regulating blade 46 with respect to the sleeve
rotational direction is made opposite from the sleeve rotational
direction. In addition, the regulating blade 46 is disposed at the
position where the rotational direction component is larger than 1/2 of
the maximum in the region in which the direction of the rotational
direction component is opposite from the sleeve rotational direction. As
a result, the force exerted on the developer at the developer stagnating
portion 48 can be made further large and even when the above carrier is
used, the carrier can be moved similarly as in the above-described
embodiments. As a result, similarly as in the above-described
embodiments, the generation of the immobile layer of the developer can be
suppressed. Other structures and functions are the same as those in First
Embodiment.

Fourth Embodiment

[0098] Fourth Embodiment of the present invention will be described with
reference to FIG. 3 and the like. In the above-described embodiments, the
absolute values of the peak values of Br of the N1 pole and the N2 pole
or the peak value interval of Br between the N1 pole and the N2 pole was
particularly regulated. However, in order to cause the rotational
direction component (Fθ) of the magnetic force to act, in the
develop to the sleeve rotational direction, on the magnetic carrier
contacting the upstream side of the regulating blade 46 with respect to
the sleeve rotational direction, the magnetic flux density half-width of
the N2 pole may also be made larger than the magnetic flux density
half-width of the N1 pole.

[0099] In this case, it is preferable that the sleeve rotational direction
interval between the magnetic flux density peak values of the S2 pole and
the N2 pole is not more than the sleeve rotational direction interval
between the magnetic flux density peak values of the S2 pole and the N1
pole. Further, the magnetic flux density peak value of the N2 pole may
preferably be not less than the magnetic flux density peak value of the
N1 pole. Incidentally, when the half-widths are regulated as described
above and Fθ at the upstream position of the regulating blade 46 is
directed in the opposite direction to the sleeve rotational direction,
the above-described peak value intervals and peak values can be
approximately set. Other structures and functions are the same as those
in First Embodiment.

Other Embodiments

[0100] The above-described embodiments can be executed by being
appropriately combined. Further, the present invention is not limited to
the above-described constitutions but may employ various constitutions in
accordance with the present invention. In addition to the constitutions,
the present invention is not limited so long as the constitution employed
is such that the gradient of the change in Br between the S2 pole and the
N2 pole is larger than the gradient of the change in Br between the S2
pole and the N1 pole.

[0101] Further, the material of the photosensitive member 1 used in the
image forming apparatus, constitutions of the developer and image forming
apparatus, and the like in the above-described embodiments are not
limited to those described above but the present invention is applicable
to various developers and image forming apparatuses. Specifically, the
color of the toner, the number of colors, the presence or absence of the
wax, the order of development with the respective toners, the number of
developer carrying members, the amount of magnetization, and the like are
not limited to those in the above-described embodiments.

[0102] Further, here, with respect to the constitution of the developing
device, in each of the above-described embodiments, the developing
chamber 41a and the stirring chamber 41b are horizontally disposed.
However, the present invention is also applicable to a developing device
in which the developing chamber 41a and the stirring chamber 41b are
vertically disposed and other developing devices with different
constitutions.

[0103] Further, the S2 pole as the developer regulating pole is not
necessarily required to be disposed upstream of the regulating blade 46
with respect to the rotational direction of the developing sleeve 44.
Further, in the case where the magnetic poles between which the
regulating blade 46 is interposed are different in polarity, the present
invention is applicable even when the magnetic pole upstream of the cut
pole S2 has the same polarity as that of the cut pole S2.

[0104] While the invention has been described with reference to the
structures disclosed herein, it is not confined to the details set forth
and this application is intended to cover such modifications or changes
as may come within the purpose of the improvements or the scope of the
following claims.